In classical field theory, use noether theorem to compute conserved currents for electromagnetic Lagrangian.
\mathcal{L} = \frac{1}{4}F_{\mu\nu}F_{\mu\nu}, \quad F_{\mu\nu}=\partial_\mu A_\nu - \partial_\nu A_\mu
For arbitrary translational symmetries, the Noether conserved current evaluates...